Method of controlling debris in an EUV light source

ABSTRACT

Disclosed is an EUV system in which a source control loop is established to maintain and optimize debris flux while not unduly affecting optimum EUV generation conditions. One or more temperature sensors, e.g., thermocouples may be installed in the vessel to measure respective local gas temperatures. The respective local temperature as measured by the one or more thermocouples can be used as one or more inputs to the source control loop. The source control loop may then adjust the laser targeting to permit optimization of debris generation and deposition while not affecting EUV production, thus extending the lifetime of the source and its collector.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a continuation of U.S. patent application Ser. No.15/593,732, filed May 12, 2017, the entire contents of which are herebyincorporated by reference herein.

FIELD

The present disclosure relates to apparatus for and methods ofgenerating extreme ultraviolet (“EUV”) radiation from a plasma createdthrough discharge or laser ablation of a target material in a vessel. Insuch applications optical elements are used, for example, to collect anddirect the radiation for use in semiconductor photolithography andinspection.

BACKGROUND

Extreme ultraviolet radiation, e.g., electromagnetic radiation havingwavelengths of around 50 nm or less (also sometimes referred to as softx-rays), and including radiation at a wavelength of about 13.5 nm, canbe used in photolithography processes to produce extremely smallfeatures in substrates such as silicon wafers.

Methods for generating EUV radiation include converting a targetmaterial to a plasma state. The target material preferably includes atleast one element, e.g., xenon, lithium or tin, with one or moreemission lines in the EUV portion of the electromagnetic spectrum. Thetarget material can be solid, liquid, or gas. In one such method, oftentermed laser produced plasma (“LPP”), the required plasma can beproduced by using a laser beam to irradiate a target material having therequired line-emitting element.

One LPP technique involves generating a stream of target materialdroplets and irradiating at least some of the droplets with one or morelaser radiation pulses. Such LPP sources generate EUV radiation bycoupling laser energy into a target material having at least one EUVemitting element, creating a highly ionized plasma with electrontemperatures of several 10's of eV.

For this process, the plasma is typically produced in a sealed vessel,e.g., a vacuum chamber, and the resultant EUV radiation is monitoredusing various types of metrology equipment. In addition to generatingEUV radiation, the processes used to generate plasma also typicallygenerate undesirable by-products in the plasma chamber which can includeout-of-band radiation, high energy ions and debris, e.g., atoms and/orclumps/microdroplets of residual target material.

The energetic radiation is emitted from the plasma in all directions. Inone common arrangement, a near-normal-incidence mirror (often termed a“collector mirror” or simply a “collector”) is positioned to collect,direct, and, in some arrangements, focus at least a portion of theradiation to an intermediate location. The collected radiation may thenbe relayed from the intermediate location to a set of optics, a reticle,detectors and ultimately to a silicon wafer.

In the EUV portion of the spectrum it is generally regarded as necessaryto use reflective optics for the optical elements in the systemincluding the collector, illuminator, and projection optics box. Thesereflective optics may be implemented as normal incidence optics asmentioned or as grazing incidence optics. At the wavelengths involved,the collector is advantageously implemented as a multi-layer mirror(“MLM”). As its name implies, this MLM is generally made up ofalternating layers of material (the MLM stack) over a foundation orsubstrate. System optics may also be configured as a coated opticalelement even if it is not implemented as an MLM.

The optical element must be placed within the vessel with the plasma tocollect and redirect the EUV radiation. The environment within thechamber is inimical to the optical element and so limits its usefullifetime, for example, by degrading its reflectivity. An optical elementwithin the environment may be exposed to high energy ions or particlesof target material. The particles of target material, which areessentially debris from the laser vaporization process, can contaminatethe optical element's exposed surface. Particles of target material canalso cause physical damage to and localized heating of the MLM surface.

In some systems H₂ gas at pressures in the range of 0.5 to 3 mbar isused in the vacuum chamber as a buffer gas for debris mitigation. In theabsence of a gas, at vacuum pressure, it would be difficult to protectthe collector adequately from target material debris ejected from theirradiation region. Hydrogen is relatively transparent to EUV radiationhaving a wavelength of about 13.5 nm and so is preferred to othercandidate gases such as He, Ar, or other gases which exhibit a higherabsorption at about 13.5 nm.

H₂ gas is introduced into the vacuum chamber to slow down the energeticdebris (ions, atoms, and clusters) of target material created by theplasma. The debris is slowed down by collisions with the gas molecules.For this purpose a flow of H₂ gas is used which may also be counter tothe debris trajectory and away from the collector. This serves to reducethe damage of deposition, implantation, and sputtering target materialon the optical coating of the collector.

The process of generating EUV light may also cause target material to bedeposited on the walls of the vessel. Minimizing target materialdeposition on the vessel walls is important for achieving an acceptablylong lifetime of EUV sources placed in production. Also, maintaining thedirection of target material flux from the irradiation site anddirectionality of power dissipation into the buffer gas is important forensuring that the waste target material mitigation system works asintended and can acceptably manage by-products associated withvaporization of the target material.

SUMMARY

The following presents a simplified summary of one or more embodimentsin order to provide a basic understanding of the embodiments. Thissummary is not an extensive overview of all contemplated embodiments andis not intended to identify key or critical elements of all embodimentsnor set limits on the scope of any or all embodiments. Its sole purposeis to present some concepts of one or more embodiments in a simplifiedform as a prelude to the more detailed description that is presentedlater.

According to one aspect, a source control loop is established tomaintain and optimize debris flux while not unduly affecting optimum EUVgeneration conditions. One or more temperature sensors, e.g.,thermocouples, may be installed in the vessel to measure respectivelocal gas temperatures. The respective local temperature as measured bythe one or more thermocouples can be used as one or more inputs to thesource control loop. The source control loop may then adjust the drivelaser targeting, i.e., targeting of the laser used to vaporize targetmaterial, to permit optimization of debris generation and depositionwhile not affecting EUV production, thus extending the lifetime of thesource and its collector.

According to one aspect there is disclosed an apparatus for generatingEUV radiation in which the apparatus includes a vessel, a laser adaptedto generate laser radiation, and a laser steering system arranged toreceive the laser radiation and adapted to steer the laser radiation toan irradiation region within the vessel. The apparatus also includes atarget material delivery system adapted to deliver target material tothe irradiation region to be irradiated by the laser, the irradiation ofthe target material by the laser generating the EUV radiation. A targetmaterial steering system coupled to the target material delivery systemfor adjusting a position of the target material within the irradiationregion. The apparatus also includes an EUV radiation metrology systemadapted to measure at least one operating parameter of the EUV radiationand to generate a first signal indicative of a value of the operatingparameter, a temperature sensor arranged at a position within the vesseland adapted to measure a temperature within the vessel at the positionand to generate a temperature signal indicative of a value of themeasured temperature, and a controller adapted to receive the firstsignal and the temperature signal and to generate a control signal basedat least in part on the measured temperature and to provide the controlsignal to at least one of the laser steering system and the targetmaterial steering system to adjust interaction of the laser radiationand the target material in the irradiation region.

The apparatus may further include an EUV optical element located withinthe vessel and wherein the position at which the temperature sensor isarranged may be gravitationally above the EUV optical element. The EUVoptical element may be a collector mirror. The temperature sensor may bearranged on an internal wall of the vessel. The temperature sensor maybe or include a thermocouple. The apparatus may also include a secondtemperature sensor arranged at a second position within the vessel andadapted to measure a second temperature within the vessel at the secondposition and to generate a second temperature signal indicative of avalue of the second measured temperature and the controller may beadapted to receive the second temperature signal and to generate thecontrol signal based at least in part on the second measuredtemperature.

According to another aspect there is disclosed an apparatus forgenerating EUV radiation, the apparatus including a vessel, a laseradapted to generate laser radiation, and a laser steering systemarranged to receive the laser radiation and adapted to steer the laserradiation to an irradiation region within the vessel. The apparatus alsoincludes a target material delivery system adapted to deliver targetmaterial to the irradiation region to be irradiated by the laser, theirradiation of the target material by the laser generating the EUVradiation. And an EUV optical element located within the vessel. A firsttemperature sensor is arranged at a first position within the vesselgravitationally above the EUV optical element and adapted to measure afirst measured temperature within the vessel at the first position andto generate a first temperature signal indicative of a value of thefirst measured temperature. A second temperature sensor is arranged at asecond position within the vessel and adapted to measure a secondtemperature of a gas within the vessel at the second position and togenerate a second temperature signal indicative of a value of the secondmeasured temperature. A controller is adapted to receive the firstsignal and the temperature signal and to generate a control signal basedat least in part on the first measured temperature and the secondmeasured temperature to provide the control signal to the laser steeringsystem to adjust an angle at which the laser radiation strikes thetarget material in the irradiation region. The EUV optical element maybe or include a collector mirror. The first temperature sensor may bearranged is on an internal wall of the vessel and the second temperaturesensor may arranged is on an internal wall of the vessel. The firsttemperature sensor may be a thermocouple and the second temperaturesensor may be a second thermocouple.

According to another aspect there is disclosed an apparatus forgenerating EUV radiation, the apparatus including a vessel, a laseradapted to generate a laser beam, and a laser steering system arrangedto receive the laser beam and adapted to direct the laser beam to andadjust a tilt of the laser beam in irradiation region within the vessel.The apparatus also includes a target material delivery system adapted todeliver target material to the irradiation region to be irradiated bythe laser beam, the irradiation of the target material by the laser beamgenerating the EUV radiation, a temperature sensor arranged at aposition within the vessel and adapted to measure a temperature withinthe vessel at the position and to generate a temperature signalindicative of a value of the measured temperature, and a controlleradapted to receive the temperature signal and to generate a controlsignal based at least in part on the value of the temperature signal andto provide the control signal to the laser steering system to adjust thetilt of the laser beam. The tilt may be adjusted to maintain thetemperature below a predetermined maximum value. The apparatus may alsoinclude a second temperature sensor arranged at a second position withinthe vessel and adapted to measure a second temperature within the vesselat the second position and to generate a second temperature signalindicative of a second value of the measured temperature and thecontroller may be further adapted to receive the second temperaturesignal and to generate the control signal based at least in part on thesecond value of the second temperature signal and to provide the controlsignal to the laser steering system to adjust the tilt of the laserbeam.

The apparatus as claimed may also include an EUV optical element locatedwithin the vessel and wherein the position at which the temperaturesensor is arranged is gravitationally above the EUV optical element. TheEUV optical element may be or include a collector mirror. The positionat which the temperature sensor is arranged is on an internal wall ofthe vessel. The temperature sensors may be or include thermocouples.

According to another aspect there is disclosed an apparatus forgenerating EUV radiation, the apparatus including a vessel, a laseradapted to generate a laser beam, and a laser steering system arrangedto receive the laser beam and adapted to direct the laser beam to andadjust a tilt of the laser beam in irradiation region within the vessel.The apparatus also includes a target material delivery system adapted todeliver target material to the irradiation region to be irradiated bythe laser beam, the irradiation of the target material by the laser beamgenerating the EUV radiation 18. The apparatus also includes a pluralityof temperature sensors arranged at respective positions within thevessel and adapted to measure a temperature within the vessel at therespective position and to generate a plurality of temperature signalsindicative of values of the measured temperatures and a controlleradapted to receive the plurality of temperature signals and to generatea control signal based at least in part on the value of the temperaturesignals and to provide the control signal to the laser steering systemto adjust the tilt of the laser beam. The apparatus may also include anEUV optical element located within the vessel and the position at whichat least one of the plurality of temperature sensors is arranged maygravitationally above the EUV optical element. Each of the plurality oftemperature sensors may be or include a thermocouple.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic, not-to-scale view of an overall broad conceptionfor a laser-produced plasma EUV radiation source system according to anaspect of the present invention.

FIG. 2 is a schematic, not-to-scale view of a portion of the system ofFIG. 1.

FIG. 3 is a diagram of the geometry of a possible interaction of a laserbeam and a droplet of target material in a system such as the system ofFIG. 1.

FIG. 4 is a not-to-scale perspective diagram showing a possiblearrangement of temperature sensors in a vessel used in a laser-producedplasma EUV radiation source system according to an aspect of the presentinvention.

DETAILED DESCRIPTION

Various embodiments are now described with reference to the drawings,wherein like reference numerals are used to refer to like elementsthroughout. In the following description, for purposes of explanation,numerous specific details are set forth in order to promote a thoroughunderstanding of one or more embodiments. It may be evident in some orall instances, however, that any embodiment described below can bepracticed without adopting the specific design details described below.In other instances, well-known structures and devices are shown in blockdiagram form in order to facilitate description of one or moreembodiments.

With initial reference to FIG. 1 there is shown a schematic view of anexemplary EUV radiation source, e.g., a laser produced plasma EUVradiation source 10 according to one aspect of an embodiment of thepresent invention. As shown, the EUV radiation source 10 may include apulsed or continuous laser source 22, which may for example be a pulsedgas discharge CO₂ laser source producing a beam 12 of radiation at 10.6μm or 1 μm. The pulsed gas discharge CO₂ laser source may have DC or RFexcitation operating at high power and at a high pulse repetition rate.

The EUV radiation source 10 also includes a target delivery system 24for delivering target material in the form of liquid droplets or acontinuous liquid stream. In this example, the target material is aliquid, but it could also be a solid or gas. The target material may bemade up of tin or a tin compound, although other materials could beused. In the system depicted the target material delivery system 24introduces droplets 14 of the target material into the interior of avacuum chamber 26 to an irradiation region 28 where the target materialmay be irradiated to produce plasma. In some cases, an electrical chargeis placed on the target material to permit the target material to besteered toward or away from the irradiation region 28. It should benoted that as used herein an irradiation region is a region where targetmaterial irradiation may occur, and is an irradiation region even attimes when no irradiation is actually occurring. The EUV light sourcemay also include a beam steering system 32 as will be explained in moredetail below in conjunction with FIG. 2.

In the system shown, the components are arranged so that the droplets 14travel substantially horizontally. The direction from the laser source22 towards the irradiation region 28, that is, the nominal direction ofpropagation of the beam 12, may be taken as the Z axis. The path thedroplets 14 take from the target material delivery system 24 to theirradiation region 28 may be taken as the X axis. The view of FIG. 1 isthus normal to the XZ plane. The orientation of the EUV radiation source10 is preferably rotated with respect to gravity as shown, with thearrow G showing the preferred orientation with respect gravitationallydown. This orientation applies to the EUV source but not necessarily tooptically downstream components such as a scanner and the like. Also,while a system in which the droplets 14 travel substantiallyhorizontally is depicted, it will be understood by one having ordinaryskill in the art the other arrangements can be used in which thedroplets travel vertically or at some angle with respect to gravitybetween and including 90 degrees (horizontal) and 0 degrees (vertical).

The EUV radiation source 10 may also include an EUV light sourcecontroller system 60, which may also include a laser firing controlsystem 65, along with the beam steering system 32. The EUV radiationsource 10 may also include a detector such as a target positiondetection system which may include one or more droplet imagers 70 thatgenerate an output indicative of the absolute or relative position of atarget droplet, e.g., relative to the irradiation region 28, and providethis output to a target position detection feedback system 62.

The target position detection feedback system 62 may use the output ofthe droplet imager 70 to compute a target position and trajectory, fromwhich a target error can be computed. The target error can be computedon a droplet-by-droplet basis, or on average, or on some other basis.The target error may then be provided as an input to the light sourcecontroller 60. In response, the light source controller 60 can generatea control signal such as a laser position, direction, or timingcorrection signal and provide this control signal to the laser beamsteering system 32. The laser beam laser beam steering system 32 can usethe control signal to change the location and/or focal power of thelaser beam focal spot within the chamber 26. The laser beam steeringsystem 32 can also use the control signal to change the geometry of theinteraction of the beam 12 and the droplet 14. For example, the beam 12can be made to strike the droplet 14 off-center or at an angle ofincidence other than directly head-on.

As shown in FIG. 1, the target material delivery system 24 may include atarget delivery control system 90. The target delivery control system 90is operable in response to a signal, for example, the target errordescribed above, or some quantity derived from the target error providedby the system controller 60, to adjust paths of the target droplets 14through the irradiation region 28. This may be accomplished, forexample, by repositioning the point at which a target delivery mechanism92 releases the target droplets 14. The droplet release point may berepositioned, for example, by tilting the target delivery mechanism 92or by shifting the target delivery mechanism 92. The target deliverymechanism 92 extends into the chamber 26 and is preferably externallysupplied with target material and a gas source to place the targetmaterial in the target delivery mechanism 92 under pressure.

Continuing with FIG. 1, the radiation source 10 may also include one ormore optical elements. In the following discussion, a collector 30 isused as an example of such an optical element, but the discussionapplies to other optical elements as well. The collector 30 may be anormal incidence reflector, for example, implemented as an MLM withadditional thin barrier layers, for example B₄C, ZrC, Si₃N₄ or C,deposited at each interface to effectively block thermally-inducedinterlayer diffusion. Other substrate materials, such as aluminum (Al)or silicon (Si), can also be used. The collector 30 may be in the formof a prolate ellipsoid, with a central aperture to allow the laserradiation 12 to pass through and reach the irradiation region 28. Thecollector 30 may be, e.g., in the shape of a ellipsoid that has a firstfocus at the irradiation region 28 and a second focus at a so-calledintermediate point 40 (also called the intermediate focus 40) where theEUV radiation may be output from the EUV radiation source 10 and inputto, e.g., an integrated circuit lithography scanner 50 which uses theradiation, for example, to process a silicon wafer workpiece 52 in aknown manner using a reticle or mask 54. The silicon wafer workpiece 52is then additionally processed in a known manner to obtain an integratedcircuit device.

The arrangement of FIG. 1 also includes a temperature sensor 34, e.g., athermocouple positioned within the chamber 26 to measure the localtemperature, i.e., temperature at the sensor, of the gas within thechamber 26. FIG. 1 shows one temperature sensor but it will be apparentthat additional temperature sensors may be used. The temperature sensor34 generates a signal indicative of the measured temperature andsupplies it as an additional input to the controller 60. The controller60 bases the control signal it supplies to the beam steering system 32at least in part on this temperature signal.

As discussed below, it has been found that controlling the offset ofbeam impingement on the droplet with respect to the center of mass ofthe droplet (“tilt”) can optimize debris control without sacrificing EUVgeneration performance. Specifically, it has been found that negativetilt (deliberately causing the beam to strike the droplet slightly toone side of the center of mass of the droplet) can minimize targetmaterial deposition in areas of the chamber 26 where it is desired toavoid deposition of target material without materially affecting EUVradiation production. It has also been determined that the temperaturedistribution in the chambers bears a correlation to the distribution oftarget material debris. The controller 60 thus uses the input from thetemperature sensor 34 as at least a partial basis to generate a controlsignal. This is in conjunction with a control loop responsible foroptimizing EUV generation. It has been determined that debris productionand EUV generation are essentially decoupled so that successful EUVproduction can be achieved with limited debris production.

Continuing to FIG. 2, it can be seen that the beam steering system 32may include one or more steering mirrors 32 a, 32 b, and 32 c. Althoughthree mirrors are shown, it is to be appreciated that more than three oras few as one steering mirror may be employed to steer the beam.Moreover, although mirrors are shown, it is to be appreciated that otheroptics such as prisms may be used and that one or more of the steeringoptics may be positioned inside the chamber 26 and exposed toplasma-generated debris. See for example U.S. Pat. No. 7,598,509 filedon Feb. 21, 2006, and titled LASER PRODUCED PLASMA EUV LIGHT SOURCE, theentire contents of which are hereby incorporated by reference herein.For the embodiment shown, each of the steering mirrors 32 a, 32 b, and32 c may be mounted on a respective tip-tilt actuator 36 a, 36 b, and 36c which may move each of the steering mirrors 32 a, 32 b, and 32 cindependently in either or both of two dimensions.

It has been noted that very small changes in Y-axis tilt of the CO₂laser beam can lead to very significant changes in target materialdeposition without affecting EUV generation. FIG. 3 is a diagram toillustrate the concept of Y tilt as applied to the geometry of theinteraction of the CO₂ laser beam 12 and the droplet 14. The Z axis isthe direction along the nominal (no Y-tilt) propagation of the laserbeam. Droplets travel along the X-axis, which is perpendicular to Z-axisand is horizontal in the global frame of reference. The Z-coordinate ofthe droplet travel path is Z=0. Y-tilt leads to the beam hittingslightly to one side of the center of the droplet as it travels throughthe beam focal spot. Thus, in the situation shown in FIG. 3, the beam 12strikes the droplet (has a focal point at) to one side of the droplet 14(below the nominal beam path or Z axis). This is described as negativeY-tilt. Y-tilt is measured as a displacement of the location the beamstrikes the droplet from the location the beam strikes the droplet inthe zero Y tilt condition. For example, a value of negative Y-tilt mighttypically be on the order of −10 microns. In FIG. 3, the droplet isshown as spherical but it will be understood that the droplet shape willnot necessarily be spherical and may assume other shapes, for example,if flattened by a prepulse. The displacement is thus measured from thecenter of mass of the droplet.

The relative orientation of CO₂ beam and the droplet controls the flowof target material debris. If the CO₂ beam is dead center on the dropletthen the target material debris tends to propagate in the direction ofbeam propagation parallel to Z axis. Shifting the center of the beamrelative to the center of the droplet causes the flux of debris to betilted, that is, to propagate with a component normal to the Z axis. Theactual Y-tilt of the beam is negligibly small compared to the tilt oftarget material debris flux caused by the laser-droplet misalignment. Anactual Y-tilt on the order of 20 microrads has been found to cause ashift in debris direction on the order of 0.1 rad or 5000 times larger.

It is not that important how the beam-droplet misalignment is achieved.It can be achieved by shifting the position of the center of the beam bysteering the beam or it can be achieved by shifting the position of thedroplet by manipulating its release point. It is also possible where thedroplet trajectory has a vertical component to achieve the desireddisplacement/misalignment by controlling the timing of droplet releasewith respect to pulse timing by itself or in conjunction with dropletdisplacement and/or laser shift.

It has been determined that the rate of target material deposition on agiven portion of the vessel for negative Y-tilt can be made markedlyless than deposition rate for positive Y-tilt. The temperatures asmeasured by temperature sensors and are indicative of the rate of targetmaterial deposition were lower for the lower deposition rate, negativeY-tilt condition than for the higher deposition rate, positive Y-tiltcondition. At the same time, the amount of Y-tilt involved (about 10microns) and did not affect EUV production. Thus at negative tilt, whichis tilting the CO₂ beam away from location of the temperature sensor,the target material deposition rate becomes very small, while at thepositive Y-tilt (towards location of the temperature sensor), the targetmaterial accumulation rate reaches has a very high value.

It is presently preferred that the distance from the plasma to thetemperature sensor location be in the range of about 200 mm to about 250mm. The temperature sensor may be a “bare” thermocouple that has a metaljunction exposed to the environment. Materials such as those making upsuch thermocouples have high recombination probability for H-radicals,and as a result this type of thermocouple measures a higher temperaturevalue which is the sum of the gas temperature and extra heating due toH-radical recombination. The other type is a “glass” thermocouple inwhich the metal junction is inserted into a glass capillary to protectit from direct contact with the environment. The recombinationprobability for H-radicals on glass is much lower (about 1000 timeslower) than on bare metal, so the glass thermocouple reads a lowervalue, determined only by gas temperature. As a practical matter,however, in the application in which the thermocouple is exposed todebris accumulation, a glass thermocouple will become coated with targetmaterial debris relatively quickly so that the difference in measuredtemperature is not significant. In the presently preferred embodimentbare thermocouples are used.

In order to create the desired control loop at least one thermocoupleshould be installed in the vessel in the areas where it is desired tominimize debris accumulation, i.e., minimize the flow of debris towardsthat area. As an example, one such area is may be the vessel wallsdirectly above the collector. Target material debris accumulation inthis area creates the risk that target material will drip onto thecollector. FIG. 4 shows an example where the thermocouples 34 arepositioned around the circumference of internal walls 44 of arotationally symmetric vessel 26 at a position between the collector 30and the intermediate location 40. FIG. 4 shows an arrangement in whichsix thermocouples 34 are used but it will be understood that fewer ormore thermocouples may be used and that different arrangements andpositioning of the thermocouples may be used. Each thermocouple ispreferably configured as a small diameter wire (less than 1 mm indiameter) that protrudes into the gas for about 2 cm from the wall 44.For such a thermocouple, even if the wire protrudes somewhat into thepath of EUV propagation, the total EUV loss will be negligible. Thesolid double arrow in FIG. 4 shows the direction of debris propagation.The outline arrows show a preferred arrangement for causing H₂ to flowaway from the collector 30. Elements 42 are scrubbers for removingcontaminants from the H₂. Arrow G indicates the direction of gravity.Also shown is a line demarking a division between the source 10 and thescanner 50.

The thermocouple temperature readings are supplied to the controller 60as inputs. FIG. 4 shows one such connection 46 but it will be understoodthat each of the thermocouples 34 is connected to supply a signal to thecontroller 60. The controller 60 then controls the beam steering system32 to adjust the beam tilt such that to minimize the readings from thethermocouples installed in the areas where it is desired to minimizedebris accumulation. In one embodiment, the control loop made up of thethermocouple, controller, and beam steering system can be conceptualizedas operating to minimize the error signal Y_(err) according to therelationshipY _(err)=Σ(T _(i,protected))/Σ(T _(i,all))

where Σ(T_(i, protected)) is the sum of temperature readings over thearea directly above collector and Σ(T_(i, all)) is the sum of allreadings around the wall circumference.

The control loop adjusts Y-tilt to minimize the error signal. The totaltilt Y_(tilt CO2) is the sum of Y-tilt value set by the main controlsystem operating to optimize EUV production, Y_(tilt EUV), and theY-tilt value correction set by the “debris loop” as described above,Y_(tilt Debris), thus:Y _(tilt CO2) =Y _(tilt EUV) +Y _(tilt Debris).

Preferably a maximum absolute value of Y_(tilt Debris) should be limitedto a predetermined value, preferably in the range of about 10 microns toabout 20 microns. Of course, the preferred value will depend on theimplementation of the “tilt.” For example, if the target is flat, it maybe possible to implement “tilt” by creating a displacement between themidpoint of the flat target and where it is struck by the beam to effecta change in the debris distribution.

It is believed that the measurement of local temperature provides usefulinformation on the local concentration of Sn. The physical equations ofconvection and diffusion apply both to heat and Sn concentration. Also,the heat source and the Sn source largely coincide at the irradiationsite. Therefore it is reasonable to infer that the measured temperatureprovides an indication of Sn concentration. This correlation is notnecessarily uniform but is sufficient to provide an input to the controlloop if care is taken to ensure measuring temperature at a locationwhere the correlation can be expected to be strong. In principle, it isnecessary only to locate the temperature sensor in a position in whichthere will be a significant signal to noise for the metrology andcontroller.

For example, it is preferred that the thermocouple be shielded fromincoming EUV radiation to prevent sensing the wrong temperature. Also,the boundary conditions at the wall may cause the correlation to breakdown. Sn concentration at a wall may approach zero if Sn is permitted orcaused to condense on the wall, and the wall may be temperaturecontrolled. It is thus preferred that the thermocouple be placed farenough away from the nearest wall to prevent the boundary conditions atthe wall from unduly distorting the temperature measurement. It ispresently preferred that the temperature be measured at about 2 cm fromthe wall, but the temperature could be measured as far as 25 cm from thewall or farther. Each thermocouple is preferably configured as a smalldiameter wire (less than 1 mm in diameter) that protrudes into the gasfor the preferred length. It also may be preferred for some applicationsto position another thermocouple at the opposite side of the chamber todevelop a measurement of temperature near the wall in the presence of ahigher or lower SN concentration.

As described above, the interaction between the incoming beam and thetarget material is controlled to affect the dispersion of targetmaterial in the chamber. The temperature at a given location is measuredas an indication of the target material concentration at that location.The measured temperature is then used as an input to a control loop tocontrol the beam/target material interaction to obtain a desired targetmaterial concentration at the location.

As an example in some systems using droplets of Sn the lasersequentially supplies two pulses to each droplet, a first pulse called aprepulse and a second pulse called a main pulse. The purpose of theprepulse is to precondition the droplet and the purpose of the mainpulse is to vaporize the droplet after it has been conditioned by theprepulse. For example, if the prepulse strikes the droplet head-on thenthe target expands flat into what is referred to as a “flat target”which will present a flat face to the main pulse which is not tilted.The position where the droplet is vaporized is preferably the primaryfocus of the collector. In other words, to obtain a good, focused imageof the vaporization event to be relayed to the scanner it is preferredto have the main pulse impact the target at the primary focus.

As described above, “tilt” may be achieved on by displacing the positionwhere the prepulse strikes the droplet. This causes the target to expandat an angle, thus resulting in a tilted target for the main pulse. Tocreate the displacement the droplet may be at the primary focus whenstruck and the laser is displaced, or the laser may be directed at theprimary focus and there is a displacement between the droplet and theprimary focus. The tilt is what determines the “tilted” debris emission.There are, however, different ways to affect the ion distribution (e.g.,target shape; target tilt, main pulse displacement). Whichever method isused, the debris pattern as measured by the thermocouples is used asfeedback in the control loop.

In other words, the beam-droplet interaction may be altered by adjustingthe pointing (tilt) of the CO2 beam. It is also possible to adjust thebeam-droplet interaction by adjusting the horizontal or verticalposition at which the beam strikes the droplet. This can be accomplishedby controlling the target delivery control system 90 to cause the targetdelivery mechanism 92 to change the release point of the droplet. It canalso be accomplished by changing the relative timing of the dropletrelease and generation of the pulse in systems in which the droplettravels vertically. Controlling the droplet/beam interaction in thismanner may have the advantage of reducing the amount of shift in theposition of the plasma caused by the operation of the control loop.

After adjustment of either Y-tilt of CO₂ beam or the position of thedroplet/beam interaction, both CO₂ beam and the droplet/beam interactionposition can be moved (tilted or shifted) simultaneously to a newlocation (without affecting mutual alignment between the two) ifadjustment in the plasma position is desired. This simultaneous shift ofY-tilt and the droplet/beam interaction position will not affect debrisflux or EUV production. If further adjustment in either of the loop isrequired, however, the process can be repeated.

In yet another embodiment, the temperature reading can be used to directthe flow of debris in a predictable way, such as collinear with theZ-axis, for example, to minimize the target material deposition on thewalls everywhere in the vessel. In this case the control loops couldoperate to minimize the value:Y _(err)=Σ(T _(i,all)).

It is also possible to use this control loop to minimize debris fluxonto the collector. In this case, the positions for the one or moretemperature sensors should be shifted towards collector (along the Zaxis). Also it is possible to install the temperature sensors in theareas which are determined to be the most sensitive indicators fordebris based on computational fluid dynamics (CFD) simulations.

Thus, by using very small and simple temperature sensors such asthermocouples in the EUV source it is possible to design a debriscontrol loop that allows for stabilizing, minimizing, and directing theentrained target material debris in the H₂ flows. The ability to directflows away from surfaces which are positioned gravitationally above thecollector could in particular be used to extend collector lifetime.

The above description includes examples of one or more embodiments. Itis, of course, not possible to describe every conceivable combination ofcomponents or methodologies for purposes of describing theaforementioned embodiments, but one of ordinary skill in the art mayrecognize that many further combinations and permutations of variousembodiments are possible. Accordingly, the described embodiments areintended to embrace all such alterations, modifications and variationsthat fall within the spirit and scope of the appended claims.Furthermore, to the extent that the term “includes” is used in eitherthe detailed description or the claims, such term is intended to beinclusive in a manner similar to the term “comprising” as “comprising”is construed when employed as a transitional word in a claim.Furthermore, although elements of the described aspects and/orembodiments may be described or claimed in the singular, the plural iscontemplated unless limitation to the singular is explicitly stated.Additionally, all or a portion of any aspect and/or embodiment may beutilized with all or a portion of any other aspect and/or embodiment,unless stated otherwise.

What is claimed is:
 1. A method of controlling generation of EUVradiation in a vessel, the method comprising the steps of: measuring atleast one operating parameter of the EUV radiation and generating afirst signal indicative of a value of the operating parameter; measuringa temperature at a position within the vessel and generating atemperature signal indicative of a value of the measured temperature;generating a control signal based at least in part on the measuredtemperature and the first signal; and providing the control signal to atleast one of a laser steering system and a target material steeringsystem to adjust an angle at which laser radiation strikes the targetmaterial in an irradiation region in the vessel.
 2. A method as claimedin claim 1 wherein the position is gravitationally above an EUV opticalelement located within the vessel.
 3. A method as claimed in claim 2wherein the EUV optical element comprises a collector mirror.
 4. Amethod as claimed in claim 1 wherein the position at which thetemperature is measured is on an internal wall of the vessel.
 5. Amethod as claimed in claim 1 wherein the step of measuring a temperatureat a position within the vessel and generating a temperature signalindicative of a value of the measured temperature is performed using athermocouple.
 6. A method as claimed in claim 1 further comprising astep of measuring a second temperature within the vessel at a secondposition and generating a second temperature signal indicative of avalue of the second measured temperature and wherein the step ofgenerating a control signal comprises generating the control signalbased at least in part on the second measured temperature.
 7. A methodof controlling generation of EUV radiation in a vessel, the methodcomprising the steps of: measuring a first temperature at a firstposition within the vessel; measuring a second temperature at a secondposition within the vessel; generating a control signal based at leastin part on the first measured temperature and the second measuredtemperature; and adjusting an angle at which laser radiation strikestarget material in an irradiation region in the vessel based at least inpart on the control signal.
 8. A method as claimed in claim 7 whereinthe first position is gravitationally above an EUV optical element inthe vessel.
 9. A method as claimed in claim 7 wherein the first positionis on an internal wall of the vessel and the second position is on theinternal wall of the vessel.
 10. A method as claimed in claim 7 whereinthe step of measuring a first temperature is performed using a firstthermocouple and the step of measuring a second temperature is performedusing a second thermocouple.
 11. A method of controlling generation ofEUV radiation in a vessel, the method comprising the steps of:generating a laser beam; generating EUV radiation in the vessel bystriking target material with the laser beam; measuring a temperature ata position within the vessel; and adjusting a tilt of the laser beambased at least in part on the temperature.
 12. A method as claimed inclaim 11 wherein the tilt is adjusted to maintain the temperature belowa predetermined maximum value.
 13. A method as claimed in claim 11wherein the position at which the temperature is measured isgravitationally above an EUV optical element located in the vessel. 14.A method as claimed in claim 13 wherein the EUV optical elementcomprises a collector mirror.
 15. A method as claimed in claim 11wherein the position at which the temperature is measured is on aninternal wall of the vessel.
 16. A method as claimed in claim 11 furthercomprising a step of measuring a second temperature at a second positionin the vessel and wherein the step of adjusting a tilt of the laser beambased at least in part on the temperature further comprises adjustingthe tilt of the laser beam based at least in part on the secondtemperature.
 17. A method as claimed in claim 16 wherein the step ofmeasuring a temperature is performed using a first thermocouple and thestep of measuring a second temperature is performed using a secondthermocouple.
 18. A method as claimed in claim 11 further comprising thesteps of: measuring a second temperature at a second position within thevessel; and adjusting a tilt of the laser beam based at least in part onthe second temperature.
 19. A method as claimed in claim 18 wherein thesecond position is arranged gravitationally above an EUV optical elementin the vessel.
 20. A method as claimed in claim 18 wherein each of thesecond temperature is measured using a thermocouple.